Essential Cell Biology 5th edition

The Breakdown and Utilization of Sugars and Fats
441
the transfer of its terminal phosphate group to ADP produces one ATP
molecule in each cycle. Like NADH, FADH 2 is a carrier of high-energy
electrons. And like NADH, FADH 2 transfers its high-energy electrons to
the electron-transport chain in the inner mitochondrial membrane. As
we discuss shortly, the movement of energy stored in these readily transferrable
electrons is subsequently used to produce ATP through oxidative
phosphorylation on the inner mitochondrial membrane, the only step in
the oxidative catabolism of foodstuffs that directly requires O 2 from the
atmosphere.
Many Biosynthetic Pathways Begin with Glycolysis or
the Citric Acid Cycle
Catabolic reactions, such as those of glycolysis and the citric acid cycle,
produce both energy for the cell and the building blocks from which many
other organic molecules are made. Thus far, we have emphasized energy
production rather than the provision of starting materials for biosynthesis.
But many of the intermediates formed in glycolysis and the citric acid
cycle are siphoned off by such anabolic pathways, in which the intermediates
are converted by a series of enzyme-catalyzed reactions into
amino acids, nucleotides, lipids, and other small organic molecules that
the cell needs. The oxaloacetate and α-ketoglutarate produced during
the citric acid cycle, for example (see Panel 13–2), are transferred from
the mitochondrial matrix back to the cytosol, where they serve as precursors
for the production of many essential molecules, such as the amino
acids aspartate and glutamate, respectively. An idea of the extent of these
anabolic pathways can be gathered from Figure 13–14, which illustrates
some of the branches leading from the central catabolic reactions to biosyntheses.
How cells control the flow of intermediates through anabolic
and catabolic pathways is discussed in the final section of the chapter.
QUESTION 13–4
Looking at the chemistry detailed in
the overview of the citric acid cycle
at the top of the first page of Panel
13–2 (p. 442), why do you suppose
it is useful to link the two-carbon
acetyl group to another, larger
carbon skeleton, oxaloacetate,
before completely oxidizing both of
the acetyl-group carbons to CO 2 ?
(See also Figure 13−12.)
GLUCOSE
glucose 6-phosphate
fructose 6-phosphate
nucleotides
amino sugars
glycolipids
glycoproteins
serine
GLYCOLYSIS
3-phosphoglycerate
dihydroxyacetone
phosphate
lipids
amino acids
pyrimidines
alanine
aspartate
other amino acids
purines
pyrimidines
heme
chlorophyll
oxaloacetate
succinyl CoA
phosphoenolpyruvate
pyruvate
CITRIC
ACID
CYCLE
citrate
α-ketoglutarate
cholesterol
fatty acids
glutamate
other amino acids
purines
Figure 13–14 Glycolysis and the citric
acid cycle provide the precursors needed
for cells to synthesize many important
organic molecules. The amino acids,
nucleotides, lipids, sugars, and other
molecules—shown here as products—in
turn serve as the precursors for many of the
cell’s macromolecules. Each black arrow
in this diagram denotes a single enzymecatalyzed
reaction; the red arrows generally
represent pathways with many steps that are
required to produce the indicated products.